The core refractive index profile of a graded-index multimode preform and the fiber made thereof is characterized mainly by three parameters. These parameters are core diameter, maximum refractive index difference between the core and the cladding, and the profile of the refractive index of the core. A graded-index profile is generated by depositing a certain number of core layers. In order to optimise the graded-index profile, it is helpful to enhance the number of single layers to minimize the refractive index structures in the radial direction. By enhancing the number of core layers, the productivity of the deposition process is decreased. Conventionally the core layers are produced by a vapour deposition process, e.g. modified chemical vapour deposition (MCVD), plasma inside/outside vapour deposition (PIVD/POVD) or outside vapour deposition (OVD).
The MCVD process uses a layer specific precursor composition to achieve a desired refractive index profile. The gases in the composition react within a hot zone to dope glass and deposit it on the inner surface of a tube. At the start of the process, a burner is located at the inlet part of the tube. There are different chemical reaction and deposition conditions at the tube opening than in the middle or at the end of the tube. The opening region has changing core diameter and refractive index profile and is called the preform taper. The preform taper is a section of the tube where the gas composition enters the tube and this section is characterized by non-uniform and non-constant optical and geometrical properties. Depending on the process parameters used, the taper can typically have a length of 20 to 40 cm including the geometric and the profile taper. Within the profile taper, the refractive index profile can be described in first approximation by the profile exponent (also referred to as the alpha value). In general, the preform and fiber parameters of the taper region are so different and uncontrolled, that it is often not possible to use the fibers obtained from this preform region.
Glass soot is generated inside of the tube by an outer heat source in the MCVD process. This soot is deposited along the inner tube wall. Due to certain reaction, transport and deposition mechanisms using a homogenous gas phase reaction, a particular deposition course is generated along the tube. This soot deposition course is called a deposition function after the soot has been consolidated to transparent glass layers. The local changing single layer thickness and single layer refractive index at the inlet part of the deposition tube results in a need to reduce the preform taper.
Information relevant to attempts to address the problems described above can be found in the references described below.
German patent DE 60000283 T2 discloses a method for making a preform having a defined refractive index profile using a controlled reactive gas composition in a chemical vapor deposition (CVD) process. In addition to the composition (concentration of dopands), the velocity of the gas is aligned to reduce deviations in the refractive index profile. The criteria for optimisation is the deviation of the measured refractive index profile from the predetermined refractive index profile. The correction of the measured refractive index profile is carried out by adjusting the composition of the reactive gases as a function of time during the deposition process. To increase the accuracy of the profile measurements and the quality of corrections, the preform measurement is carried out at certain axial tube positions and angles. Additional preforms can be used to calculate a mean deviation.
The method described in DE 60000283 T2 yields unsatisfying results because the correlation of local and temporal profile deviations and the composition of the reactive gases is not acceptably precise. The mass flow controller typically used in dosing units bears only discrete adjusting possibilities and often has a time delay of some seconds. This is typically considered to be quite slow. In addition, there are effects on concentration mixing starting in the origin of the precursors, typically an evaporator, over the pipework up to the deposition area. By these mixing effects, mostly dopand diffusion, turbulent gas flow in the pipework, the modification of the gas composition is not sharp and the resulting profile correction is locally imprecise.
Another method for a correction of the refractive index profile based on preform as well as fiber profile measurements is described in U.S. published application U.S. 2011/0044596 A1. The given index profile is defined on the preform or based on the profile of the fiber. In this method, deviations of the refractive index profile are converted into changes of gas compositions and used in the CVD-process. The reaction time in the hot zone can be influenced by the velocity of the gases. Since the conversion of refractive index deviations to changes in the volume of the reacting gases can only be approximated, it is necessary to run several iteration cycles to obtain the desired refractive index profile.
In United Kingdom patent GB 2118165 A, a method to reduce the geometrical taper of preforms is described. A smaller geometrical taper is essential for good fiber quality but is not sufficient. A non-linear course of the support speed along the tube length is disclosed, which is optimised in an iterative manner.
Another known method for the profile correction of preforms built up by single layers in an inside/outside deposition process includes the variation of the gas flows of the precursor halides to reduce systematic deviations of single layer thickness and/or concentration. It has been shown that by controlling the gas flows, only locally indistinct layer thickness and refractive index changes can be employed. This is based on the time delay of the mass flow controllers, the mixing effects in the pipework from the evaporator to the reaction zone and turbulent gas flows through the reaction tube system. It is generally difficult to achieve locally precise changes in refractive index or layer thickness.
There remains a need for improved methods for the manufacture of graded-index multimode preforms particularly to reduce the front preform taper range